Elevator cars typically have a platform on top of which passengers and/or goods can be transported. The platform typically forms the load bearing structure of the cabin floor. It may be covered with a decorative panel but this is not always necessary as the platform itself may comprise a decorative surface plate. The cabin is a box-like structure with walls, ceiling, a floor and possibly door(s), which together define the interior space for receiving the load to be transported. The cabin may be a self-carrying structure and form as such the complete elevator car, or alternatively the cabin can be mounted on a car frame via the platform. In prior art, there are numerous different platform solutions. Usually, the platform is installed in one piece. That is, a large platform, which extends across the cabin from one side to the other both in the width-direction and depth-direction, is installed to form the platform of the elevator car. The problem with this type of installation is that the elevator car is difficult and slow to assemble. Large parts are heavy to move and require more space. These problems are met especially when the elevator car is intended to be assembled inside the hoistway. In the aforementioned case, where the platform is mounted in single piece, the platform has been previously assembled in the factory from numerous parts. As some of these known platforms are assembled from several pieces at the factory, it is possible that the platform could be assembled in the hoistway. However, these known platform structures have not been well suitable in practice for being assembled in the hoistway, because an accurate assembly typically requires use of complicated welding equipment and handling of large amount of components. Also such solutions have been proposed where several larger modules are connected to each other in the hoistway to form a platform. However, in the known solutions the modules have been complicated to fix to each other rigidly. In particular, they have not allowed forming of the platform of several smaller platform modules such that the platform modules together form a platform with upper face area which is the sum of the upper face areas of the platform modules.